Merge branch '2023-01-10-env-mmc-improvements-and-corrections'

Merge branch '2023-01-10-env-mmc-improvements-and-corrections'

To quote the author:

Update in U-Boot env mmc backend with several cosmetic changes or
corrections and 2 new features:

1/ CONFIG_ENV_MMC_USE_DT = no more use CONFIG_ENV_OFFSET
   in the mmc ENV backend when this config is activated.

   Requested by the STM32MP STMicroelectronics boards which activate
   several ENV_IS_IN_XXX; the value of CONFIG_ENV_OFFSET is invalid for
   SD-Card / eMMC boot; this offset should only used in SPIFlash backend
   (sf.c) for SPI-NOR boot.

   If this offset is used on mmc backend, when partition name in GPT is
   not aligned with  U-Boot DT: "u-boot,mmc-env-partition", the behavior
   is difficult to debug: a partition is corrupted on 'env save' command.

2/ selects the GPT env partition by the "u-boot-env" type GUID introduced
   by the commit c0364ce1c695 ("doc/README.gpt: define partition
   type GUID for U-Boot environment")

   This feature can also avoid issue when 'u-boot-env' partition name
   change in GPT partitioning but not in the U-Boot DT with
   "u-boot,mmc-env-partition"